Fig. 6. Mucous canals of head with branches of the ramus ophthalmicus growing towards them. Stage between O and P. Zeiss a a, ocul. 2. Fig. 7. Junction of a nerve and mucous canal. Stage P. Zeiss D, ocul. 2. Fig. 8. Longitudinal and horizontal section through the muscle-plates and adjoining structures at a stage intermediate between L and M. The section is intended to shew the gradual conversion of the cells of the somatic layer of muscle-plates into muscles. Fig. 9. Longitudinal section through the notochord and adjoining parts to shew the first appearance of the cartilaginous notochordal sheath which forms the vertebral centra. Stage N. Fig. 10. Transverse section through the tail of an embryo of stage P to shew the coexistence of the rib-process and hæmal arches in the first few sections behind the point where the latter appear. Zeiss C, ocul. 1. Fig. 11. Transverse section through the centre of a caudal vertebra of an embryo somewhat older than Q. It shews (1) the similarity between the arch-tissue and the hyaline tissue of the outer layer of the vertebral centrum, and (2) the separation of the two by the membrana elastica externa[2] (_mel_). It shews also the differentiation of three layers in the vertebral centrum: vide p. 374. Footnote 2: The slight difference observable between these two tissues in the arrangement of their nuclei has been much exaggerated by the engraver. EXPLANATION OF PLATE 14. (X. p. 378.) _This Plate illustrates the Formation of the Spinal Nerves._ COMPLETE LIST OF REFERENCE LETTERS. _ar._ Anterior root of a spinal nerve. _ch._ Notochord. _com._ Commissure connecting the posterior roots of the spinal nerves. _i._ Mesoblastic investment of spinal cord. _mp._ Muscle-plate. _n._ Spinal nerve. _nc._ Neural canal. _pr._ Posterior root of a spinal nerve. _spg._ Ganglion on posterior root of spinal nerve. _v.r._ Vertebral rudiment. _w._ White matter of spinal cord. _y._ Point where the spinal cord became segmented off from the superjacent epiblast. Figs. 1, 2, and 3. Three sections of a Pristiurus embryo belonging to stage I. Fig. 1 passes through the heart, fig. 2 through the anterior part of the dorsal region, fig. 3 through a point slightly behind this. (Zeiss CC, ocul. 2.) In fig. 3 there is visible a slight proliferation of cells from the dorsal summit of the neural canal. In fig. 2 this proliferation definitely constitutes two club-shaped masses of cells (_pr_)--the rudiments of the posterior nerve-roots,--both attached to the dorsal summit of the spinal cord. In fig. 1 the rudiments of the posterior roots are of considerable length. Fig. 4. Section through the dorsal region of a Torpedo embryo slightly older than stage I, with three visceral clefts. (Zeiss CC, ocul. 2.) The section shews the formation of a pair of dorsal nerve-rudiments (_pr_) and a ventral nerve-rudiment (_ar_). The latter is shewn in its youngest condition, and is not distinctly cellular.